The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical x-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into an axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The x-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces x-rays upon impact with the anode.
In an x-ray tube device, the induction-coupled rotor in vacuum comprises a copper outer tubing bonded to a carbon steel inner tubing. Current production methods of joining the dissimilar metals wall-to-wall for the x-ray tube application involves low melting alloy brazing or casting of molten copper onto steel material. Current practice uses a 100-150 microinch thickness of gold plating on steel tubing, which is then inserted into copper outer tubing with a minimum clearance. The assembly is also inserted into cylindrical molybdenum mold (thick wall) and set on one tubing end in a flat pan. The pan is then put on a conveyor belt that runs through a furnace at a nominal temperature of 1065.degree. C. Since copper melts at 1083.degree. C. and gold at 1063.degree. C., a margin of the operating temperature is small. During heating in hydrogen atmosphere, for reducing surface oxidation and enhancing cleanliness, a low melting alloy starts forming at 955.degree. C. from the interdiffusion between copper and gold. As the liquid is forming, the pressure built between the steel and copper tubing squeezes some of the gold-copper liquid metal to the ends of the tubing. Some of the gold atoms diffuse into solid copper and steel, forming a concentration gradient in each metal. During cooling, a thin layer of remaining liquid, mainly gold-copper alloy, in contact with steel and copper tubing surfaces freezes, forming a metallurgical bonding between the copper and steel tubings Therefore, the current practice is a brazing operation which involves a liquid phase.
Brazing with gold plating interlayer has created a very expensive step in the fabrication of x-ray tubes. This is primarily due to the high costs of gold material and plating chemicals and their handling charges. Furthermore, in spite of the increased cost, the brazing with gold plating interlayer does not provide a high temperature resistant joint for x-ray tube applications that require high thermal performance of the rotor.
It would be desirable then to have a new method of bonding the dissimilar metals of an x-ray tube which employs a low cost interlayer material to yield a strong bond at elevated temperatures.